1. Field of the Invention
The present invention relates to an image signal processing method, and more particularly, to an image signal processing method based on an offset processing mechanism.
2. Description of the Prior Art
The image signal of a television can be a video-mode signal or a film-mode signal. Based on the video standards created by the National Television Standards Committee (NTSC), the image signal is displayed on a television with a speed of 60 frames per second. However, a speed of 24 frames per second is normally applied to generate a camera signal while shooting a film. That is, when the camera signal is played on a television, the camera signal having 24 frames per second should be converted into a film-mode signal having 60 fields per second by a 3:2 pull-down technology so as to comply with the NTSC video standards.
Please refer to FIG. 1, which is a schematic diagram showing an operation for converting frames into fields based on the 3:2 pull-down technology. As shown in FIG. 1, a frame A is dismantled to generate a field 0 (even field AE), a field 1 (odd field AO) and a field 2 (even field AE), a frame B is dismantled to generate a field 3 (even field BO) and a field 4 (even field BE), and so on, for converting a camera signal having 24 frames per second into a film-mode signal having 60 fields per second.
When an image input signal such as a video-mode signal or a film-mode signal, having 60 fields per second, is forwarded to be displayed on a television, the image input signal should be de-interlaced for generating an image output signal having 60 frames per second in advance, and then the generated frames are sequentially displayed. However, the method for de-interlacing a video-mode signal is different from the method for de-interlacing a film-mode signal. Consequently, while displaying an image input signal, the image input signal should be first identified as a video-mode signal or a film-mode signal so as to apply a proper method for de-interlacing the image input signal. In other words, if an improper method is applied to de-interlace the image input signal due to a signal-mode misjudgment of the image input signal, the image display quality will be degraded significantly.
In order to identify the signal mode of an image input signal, a plurality of motion values δi are first calculated, and then the plurality of motion values δi are compared with a preset threshold for generating a plurality of field variation judge values, which are referred to as JV values hereinafter. The plurality of JV values develop to be a field variation judge value series, which is referred to as a JV series hereinafter. When a motion value δi is greater than the preset threshold, the corresponding JV value is 1. Alternatively, when a motion value δi is not greater than the preset threshold, the corresponding JV value is 0. That is, the JV value having a value of 0 means that the ith field and the (i+2)th field for generating the corresponding motion value δi are substantially identical fields.
Please continue referring to FIG. 1. When the image input signal is a film-mode signal, the motion value δ0, calculated based on a field 0 and a field 2, is not greater than the preset threshold in that both the field 0 and the field 1 are the even field AE of the frame A, and therefore the corresponding JV value is 0. Similarly, the motion value δ5, calculated based on the field 5 and field 7, is not greater than the preset threshold in that both the field 5 and the field 7 are the odd field CO of the frame C, and therefore the corresponding JV value is also 0. However, the motion values δ1˜δ4 and δ6˜δ8 are calculated based on different fields, and therefore the corresponding JV values are all 1. As shown in FIG. 1, the JV series, corresponding to the film-mode signal, is developed based on a periodical film-mode cadence sequence “01111” repeatedly. In view of that, the image input signal is identified as a film-mode signal when the periodical film-mode cadence sequence is detected.
Please refer to FIG. 2, which is a schematic diagram showing a prior-art JV series generation method, having time along the abscissa. In the prior-art JV series generation method, the motion values are compared with a fixed threshold for generating the JV series. All the motion values in FIG. 2 are corresponding to a film-mode signal, and therefore the JV series is supposed to comprise the periodical film-mode cadence sequences repeatedly. However, if the JV series is generated based on a fixed threshold THf1, errors occur to the JV series during the intervals ΔT2 and ΔT3 in that the motion values during the intervals ΔT2 and ΔT3 are relatively low and parts of the JV values are misjudged to be 0. Besides, errors also occur to the JV series during the intervals ΔT4 and ΔT5 in that the motion values during the intervals ΔT4 and ΔT5 are relatively high and parts of the JV values are misjudged to be 1. When parts of the JV values are generated incorrectly, the image input signal will be misidentified as a video-mode signal, and a method for de-interlacing a video-mode signal is applied to de-interlace the image input signal with film-mode, which results in a lower image display quality.
Alternatively, if the JV series is generated based on a lower fixed threshold THf2 for processing lower motion values, errors still occur to the JV series during the intervals ΔT4 and ΔT5 having high motion values. On the other hand, if the JV series is generated based on a higher fixed threshold THf3 for processing higher motion values, errors still occur to the JV series during the intervals ΔT2 and ΔT3 having low motion values. That is, while performing the prior-art JV series generation method based on a fixed threshold, parts of the JV values are likely to be generated incorrectly due to high motion value variation between different intervals, and an improper de-interlacing method may be applied to de-interlace the image input signal, which will degrade the image display quality significantly.